In Windows you change the brush size with Ctrl+Alt+Mouse wheel.
How to do it in macOS environment? I tried them all, but without finding the solution.
I assume this works with MacOS (dangerous assumption)
You need to go into the Preferences menu which in a Mac is in that left hand (Gimp?) drop down menu.
Then bottom of the list Input Devices → Input Controllers
Select Main Mouse Wheel to bring up the Configure Input Controller menu
Select Scroll Up which should be blank
Click on Edit Event which brings up a Select Action for Event… menu
Scroll down the Action list to the Tool’s Size section and choose one of the options increase by 1 / …by 10 …
OK that and check that the correct entry is against the Scroll up entry
…repeat the process for Scroll Down…decrease by 1 / …by 10…
Click the Close button for the Configure Input controller menu
OK the Preferences menu to close
Try it out.
Just about get that into a 1 minute animation, but it is linux not MacOS.
It works, it works very well!
I found that the setting to change the size already existed by default: Cmd+Alt+Mouse wheel, but it doesn’t work. Instead, assigning as you taught me Alt+Mouse wheel (previously assigned to opacity), works perfectly.